In the about two decades since their introduction, desktop computers have become commonplace. There are now more than 80 million top computers of different types in use around the world. IBM-compatibles and Apple Macintoshes, which are used for business applications, comprise about 90% of the machine sold today. These are collectively known as personal computers, but IBM compatibles are usually called PCs, while Apples are called Macs. The remaining 10% of the market is occupied by higher powered and more expensive machines, generally called engineering "workstations", from Sun Microsystems, Hewlett Packard, Silicon Graphics, Digital Corporation (DEC), etc.
The Macs and the types of workstations are proprietary systems that each made by a single manufacturer. However, PCs have an open architecture. This made possible the entry of a large number of different manufacturers, which lead to the explosive growth of the PC market. However, this with in the business of developing and selling PCs and peripherals has brought about an extreme level of corn. Because of the very large number of competitors the high degree of similarity between competing products, the market has become cost driven. Many computer consumers buy the lowest cost machine in a given central processing unit (CPU) class, with little or no brand preference. Consequently, all vendors must compete vigorously on price. The price/performance ratio has become the most important factor in the PC market.
The main component of a typical PC is the motherboard (MB), or a large circuit board that holds the CPU, such as an Intel 80486, random access memory (RAM), control chips, and buffer chips. Expansion slots, which are elongated electrical connectors, on the MB accept peripheral devices such as video controller boards or cards, I/O cards, hard/floppy drive controller cards, network adapters, modems, and so forth. These cards usually control other devices such as monitors, hard and floppy drives, scanners, and more.
A bus structure handles the communication between the separate chips and peripherals. This bus structure includes individual conductive traces (signal paths) on the circuit boards that link the individual components, control chips for controlling the transfer of the signals, and connectors for plugging in the various expansion cards on the MB. The present invention relates closely to such bus structures or architectures.
Early digital computers manipulated 4-bit binary information, or words with a length of 4 decimal places. This communicated a maximum quantity of data of up to 16D, where the D simply indicates a base-ten decimal number. These gave way to more powerful machines with 8-bit communication capability. Today 32-bit data transfer is common. Longer word lengths allow more words or data to be manipulated, and increase the data transfer rate between the various components within a computer.
The word length of the binary code also strongly indicates the bus width, or the number of signal paths on the bus. In most PCs, each bit of information requires a separate signal path for its transfer within the machine. For example, in a computer capable of addressing memory with 32-bit addresses and transferring data with 32-bit word lengths, 64 traces are needed for the address bus and the data bus alone, because the two are usually kept separate. However, many additional paths are needed for other functions, such as I/O cycles, interrupt requests (IRQs), etc.
The number of pins on the IC (integrated circuit) packages that connect to the bus is directly related to the bus width. For example, if there are to be 64 paths for the address bus and the data bus, then every IC that communicates with the bus must have 64 pins to connect to just that part of the bus structure. However, most machines have more than 64 paths in the bus and more than 64 pins in the ICs. For example, the IBM-compatible ISA (Industry Standard Architecture) bus has 96 signals, each with a dedicated trace. The large number of traces makes the layouts on the MB and peripherals very complicated and expensive. Among the 96 signals, at least 80 are typically active simultaneously, each requiring a current of 24 milliamps (ma), and switching at a speed of about 8 MHz. This amounts to a high frequency loading of 1.6 amps, which causes heat buildup and radio frequency interference (RFI) of concern to the Federal Communications Commission.
As the clock speeds of PCs increase, the physical length of the signal paths has become another important consideration in bus design. Longer paths can cause longer delays, because signals propagate through the paths at finite speeds. Delays on the order of nanoseconds can have significant impact in the performance of high frequency machines.
Another important consideration in computer design is compatibility. The vast majority of new PC hardware must work with the installed base of existing hardware and software, otherwise the new machines and devices will not gain acceptance in the market place. Therefore, hardware manufacturers expend a great deal of effort to ensure that their new and advanced, products remain compatible with most existing equipment.
In some general-purpose computer architectures, such as DEC's Turbochannel bus used in powerful workstations, the address and data buses are combined into a single bus. The address and data signals are multiplexed, or alternated, so that they can both be carried on the same group of 32 signal paths. Many benefits accrue as a result of multiplexing the address and data buses. For example, the pin numbers on ICs and motherboard layout congestion are greatly reduced. The reduced congestion allows shorter signal paths, so that signal delays are also reduced. The reduction in the size and complexity of many components provide cost savings. However, multiplexed bus architectures have so far been limited typically to workstations.
Power consumption, pin count, complexity, path length and so forth, have critical application in portable computers, such as notebook and laptop computers. What is needed, especially to advance to power, useful life, and cost issues for such computers is bus system and structure that radically reduces pin count, power consumption, complexity, and buffer requirements.